1. Field of the Invention
The presently disclosed subject matter relates to a relief printing plate (letterpress printing plate) manufacturing method, relief printing plate creating apparatus, a non-transitory recording medium storing a computer readable program code for executing a process of the method, and a computer program product storing executable code for the method and, in particular, to the structure of a relief printing plate suitable for flexo printing and its plate-making technology.
2. Description of the Related Art
In flexo printing which is one kind of relief printing, a flexible, resilient plate material such as a rubber sheet or photo-curable resin sheet is used. And, a projection (a relief) forming a projecting shape for dot printing is formed on the surface of the plate material according to the details of the printing image. To a top part of the relief on a flexo printing plate, ink is provided from an anilox roller. The ink provided to this top part of the relief is transferred to a printing body for printing. Since the plate material has resilience in flexo printing, flexo printing is advantageously suitable for printing on a material with some asperities on the surface of the printing body (for example, corrugated cardboard).
On the other hand, since the plate material is soft, disadvantageously, the relief is deformed by printing pressure to increase the thickness of dots (to increase dot gain). And, superfluous ink is pushed from the top face of the relief to the surroundings to increase the thickness of dots (cause a marginal phenomenon). Also, an increase in thickness of dots disadvantageously causes deterioration of granularity (fluctuations in density representing roughness of an image). In particular, problems arise, such as a decrease in reproducibility of fine lines, an increase in thickness of characters, a decrease in tone reproducibility in a highlighted area, and an occurrence of unevenness (lack in uniformity) of printing density in a halftone dot part and a filled-in part.
Japanese Patent Application Laid-Open No. 2009-234255 discloses a method in which, for the purpose of fine laser engraving of a projection on a plate, light power for engraving an area adjacent to the projection to be left as projecting is suppressed to suppress engraving on the upper surface of the projection and also light power for its outside adjacent area is increased to steepen the gradient of the side surface of the projection.
Japanese Patent Application Laid-Open No. 2002-178654 and Japanese Patent Application Laid-Open No. 2009-286113 each disclose a structure in which a depression is provided on the top face of the relief in view of improving ink transferability in a filled-in part and mitigating a marginal phenomenon.
Japanese Patent Application Laid-Open No. 2003-043672 discloses that an average roughness of the plate surface is 0.1 μm to 0.6 μm in view of improving ink transferability in a filled-in part.